0. Department of Computer Science National Tsing Hua University
Assembler Basics
Department of Computer Science
National Tsing Hua University

1. Today’s Topic Assembler: Basic Functions
Section 2.1 of Beck’s “System Software” book.
Reading Assignment: pages

2. Role of Assembler Object Source Assembler Code Linker Program
Executable
Code
Loader

3. Example Program Fig. 2.1 (1/4)
Purpose
Reads records from input device (code F1) and store in BUFFER
Copies them to output device (code 05)
At the end of the file, writes an extra EOF on the output device, then RSUB to the operating system
Data transfer (RD, WD)
End of each record is marked with a null character
End of the file is indicated by a zero-length record
Subroutines (JSUB, RSUB)
RDREC, WRREC
Save link register first before nested jump

4. Example Program Fig. 2.1 (2/4)
5 COPY START 1000 LOAD PROG AT LOC 1000
10 FIRST STL RETADR SAVE RETURN ADDRESS
15 CLOOP JSUB RDREC READ INPUT RECORD
20 LDA LENGTH TEST FOR EOF (LENGTH = 0)
25 COMP ZERO
30 JEQ ENDFIL EXIT IF EOF FOUND
35 JSUB WRREC WRITE OUTPUT RECORD
40 J CLOOP LOOP
45 ENDFIL LDA EOF INSERT END OF FILE MARKER
50 STA BUFFER
55 LDA THREE SET LENGTH = 3
60 STA LENGTH
65 JSUB WRREC WRITE EOF
70 LDL RETADR GET RETURN ADDRESS
75 RSUB RETURN TO CALLER
80 EOF BYTE C’EOF’
85 THREE WORD 3
90 ZERO WORD 0
95 RETADR RESW 1
100 LENGTH RESW 1
105 BUFFER RESB BYTE BUFFER AREA

5. Example Program Fig. 2.1 (3/4)
110 .
115 . SUBROUTINE TO READ RECORD INTO BUFFER
120 .
125 RDREC LDX ZERO CLEAR LOOP COUNTER
130 LDA ZERO CLEAR A TO ZERO
135 RLOOP TD INPUT TEST INPUT DEVICE
140 JEQ RLOOP LOOP UNTIL READY
145 RD INPUT READ CHARACTER INTO A
150 COMP ZERO TEST FOR END OF RECORD
155 JEQ EXIT EXIT LOOP IF EOR
160 STCH BUFFER,X STORE CHAR IN BUFFER
165 TIX MAXLEN LOOP UNLESS MAX LENGTH
170 JLT RLOOP HAS BEEN REACHED
175 EXIT STX LENGTH SAVE RECORD LENGTH
180 RSUB RETURN TO CALLER
185 INPUT BYTE X’F1’ CODE FOR INPUT DEVICE
190 MAXLEN WORD 4096

6. Example Program Fig. 2.1 (4/4)
195 .
200 . SUBROUTINE TO WRITE RECORD FROM BUFFER
205 .
210 WRREC LDX ZERO CLEAR LOOP COUNTER
215 WLOOP TD OUTPUT TEST OUTPUT DEVICE
220 JEQ WLOOP LOOP UNTIL READY
225 LDCH BUFFER,X GET CHAR FROM BUFFER
230 WD OUTPUT WRITE CHARACTER
235 TIX LENGTH LOOP UNTIL ALL CHAR
240 JLT WLOOP HAVE BEEN WRITTEN
245 RSUB RETURN TO CALLER
250 OUTPUT BYTE X’05’ CODE FOR OUTPUT DEVICE
255 END FIRST

7. Assembler Directives Pseudo-Instructions Basic assembler directives
Not translated into machine instructions
Providing information to the assembler
Basic assembler directives
START
END
BYTE
WORD
RESB
RESW

8. Functions of a Basic Assembler
Mnemonic code (or instruction name)  opcode
Symbolic operands (e.g., variable names)  addresses
Choose the proper instruction format and addressing mode
Constants  Numbers
Output to object files and listing files

9. Example Program & Object Code (1/3)
Line Loc Source statement Object code
COPY START 1000
FIRST STL RETADR
CLOOP JSUB RDREC
LDA LENGTH
COMP ZERO
30 100C JEQ ENDFIL
35 100F JSUB WRREC
J CLOOP 3C1003
ENDFIL LDA EOF 00102A
STA BUFFER 0C1039
55 101B LDA THREE 00102D
60 101E STA LENGTH 0C1036
JSUB WRREC
LDL RETADR
RSUB 4C0000
80 102A EOF BYTE C’EOF’ 454F46
85 102D THREE WORD
ZERO WORD
RETADR RESW 1
LENGTH RESW 1
BUFFER RESB 4096

10. Example Program & Object Code (2/3)
110 .
SUB TO READ RECORD INTO BUFFER
120 .
RDREC LDX ZERO
C LDA ZERO
F RLOOP TD INPUT E0205D
JEQ RLOOP 30203F
RD INPUT D8205D
COMP ZERO
B JEQ EXIT
E STCH BUFFER,X
TIX MAXLEN 2C205E
JLT RLOOP 38203F
EXIT STX LENGTH
A RSUB 4C0000
D INPUT BYTE X’F1’ F1
E MAXLEN WORD

11. Example Program & Object Code (3/3)
195 .
SUB TO WRITE RECORD FROM BUFFER
205 .
WRREC LDX ZERO
WLOOP TD OUTPUT E02079
JEQ WLOOP
A LDCH BUFFER,X
D WD OUTPUT DC2079
TIX LENGTH 2C1036
JLT WLOOP
RSUB 4C0000
OUTPUT BYTE X’05’ 05
255 END FIRST

12. Examples Mnemonic code (or instruction name)  opcode Examples:
STL 1033 
LDA 1036 

13. Symbolic Operands
We are not likely to write memory addresses directly in our code
Instead, we will define variable names
Other examples of symbolic operands:
Labels (for jump instructions)
Subroutines
Constants

14. Converting Symbols to Numbers
Isn’t it straightforward?
Isn’t it simply the sequential processing of the source program, one line at a time?
Not so, if we have forward references  we don’t know the value of the symbol, because it is defined later in the code
COPY START 1000 …
LDA LEN
LEN RESW 1

15. Two-Pass Assembler Pass 1 Pass 2
Assign addresses to all statements in the program
Save the values assigned to all labels for use in Pass 2
Perform some processing of assembler directives
Pass 2
Assemble instructions by translating opcode and symbolic operands
Generate data values defined by BYTE, WORD
Perform processing of assembler directives not done in Pass 1
Write the object program and the assembly listing

16. Two-Pass Assembler From input line: LABEL, OPCODE, OPERAND
Operation Code Table (OPTAB)
Symbol Table (SYMTAB)
Location Counter (LOCCTR)
Source
program
Intermediate
file
Object
code
Pass 1
Pass 2
OPTAB
SYMTAB
SYMTAB

17. OPTAB (Operation Code Table)
Content
Mnemonic, machine code (instruction format, length) etc.
Characteristic
Static table
Implementation
Array or hash table, easy for search

18. SYMTAB (Symbol Table) Content Characteristic Implementation
Label name, value, flag, (type, length) etc.
Characteristic
Dynamic table (insert, delete, search)
Implementation
Hash table, non-random keys, hashing function
COPY 1000
FIRST
CLOOP 1003
ENDFIL 1015
EOF 1024
THREE 102D
ZERO 1030
RETADR 1033
LENGTH 1036
BUFFER 1039
RDREC 2039

19. Two Pass Assembler – Pass 1
read first input line
if OPCODE = ‘START’ then
save #[OPERAND] as starting address
initialize LOCCTR to starting address
write line to intermediate file
read next input line
else initialize LOCCTR to 0
while OPCODE  ‘END’ do
if this is not a comment line then
if there is a symbol in the LABEL field then
search SYMTAB for LABEL
if found then
set error flag (duplicate symbol)
else
insert (LABEL, LOCCTR) into SYMTAB

20. Two Pass Assembler – Pass 1
search OPTAB for OPCODE
if found then add 3 (instruction length) to LOCCTR
else if OPCODE = ‘WORD’ then
add 3 to LOCCTR
else if OPCODE = ‘RESW’ then
add 3 * #[OPERAND] to LOCCTR
else if OPCODE = ‘RESB’ then
add #[OPERAND] to LOCCTR
else if OPCODE = ‘BYTE’ then
find length of constant in bytes
add length to LOCCTR
else set error flag (invalid op code)
write line to intermediate file
read next input line // end while
write last line to intermediate file
save (LOCCTR – starting address) as program length

21. Two Pass Assembler – Pass 2
read first input line from intermediate file
if OPCODE = ‘START’ then
write listing line
read next input line
write Header record to object program
initialize first Text record
while OPCODE  ‘END’ do
if this is not a comment line then
search OPTAB for OPCODE
if found then
if there is a symbol in OPERAND field then
search SYMTAB for OPERAND
if found then store symbol value as operand address
else store 0 as operand address and set error flag
(undefined symbol)

22. Two Pass Assembler – Pass 2
else store 0 as operand address
assemble the object code instructions
else if OPCODE = ‘BYTE’ or ‘WORD’ then
convert constant to object code
if object code will not fit into the current Text record then
write Text record to object file
initialize new Text record
add object code to Text record
write listing line
read next input line
write last Text record to object file
write End record to object program
write last listing line

23. Object Program Header Text End Col. 1 H Col. 2~7 Program name
Col. 8~ Starting address (hex)
Col Length of object program in bytes (hex)
Text
Col.1 T
Col.2~7 Starting address in this record (hex)
Col. 8~9 Length of object code in this record in bytes (hex)
Col. 10~69 Object code ( )/6=10 instructions
End
Col.1 E
Col.2~7 Address of first executable instruction (hex)
(END program_name)

24. Fig. 2.3
H COPY A
T E
T 00101E 15 0C C F
T E E0205D 30203F D8205D …
T C C0000 F E …
T C
E starting address

25. 1036
xxxxxx
BYTE C’EOF’
WORD 3
WORD 0
RESW 1
1033
xxxxxx
1030
000000
STL RETADR
JSUB RDREC
LDA LENGTH
COMP ZERO
JEQ ENDFIL
LDL RETADR
RSUB
‘E’ ‘O’ ‘F’ 3
102D
000003
102A
454F46
1027
4C0000
1024
081033 …
100C
301015
1009
281030
1006
001036
1003
482039
1000
141033 …

26. One-Pass Assemblers
Forward references can be resolved in One-Pass Assemblers too!
Add a linked list to the Symbol Table to keep track of unresolved references. (See p.95)
We will discuss 1-pass assembler again (Section 2.4.1)